To understand why the maximum allowable settlement for a raft is 20 mm, let's first define what settlement is in the context of civil engineering.

Settlement refers to the downward movement of the soil or foundation upon which a structure is built. It occurs due to various factors such as the weight of the structure, the consolidation of the soil, and the presence of water. Settlement can be uniform or differential.

Uniform settlement occurs when the entire structure settles evenly, while differential settlement happens when different parts of the structure settle at different rates. Differential settlement is considered undesirable because it can lead to structural damage, such as cracks in the walls or uneven floors.

In the given question, the maximum settlement allowed for the raft is 50 mm. This means that the entire structure can settle by a maximum of 50 mm without causing any significant harm. However, to ensure the stability and integrity of the structure, the maximum allowable differential settlement is limited to 20 mm.

The reason for this limitation is to prevent excessive differential settlement, which can result in structural distortion, uneven load distribution, and potential failure. By limiting the maximum differential settlement to 20 mm, engineers can ensure that the structure remains relatively stable and safe for its intended use.

It's worth noting that the specific maximum allowable settlement and differential settlement limits may vary depending on the type of structure, soil conditions, and design considerations. In some cases, stricter limits may be imposed to ensure the safety and functionality of the structure.

In summary, the maximum settlement of 50 mm permitted for a raft refers to the overall settlement of the structure, while the maximum allowable differential settlement of 20 mm ensures that differential settlement does not exceed this limit to maintain structural stability and integrity.